Although applicable in principle to any desired bipolar transistors the present invention and the problem area on which it is based are explained with regard to DPSA transistors.
US 2002/0096742 discloses the introduction of a carbon-doped subcollector layer for reducing mechanical stresses and dislocations.
JP 2000-2162168 A discloses the introduction of a carbon-doped intermediate layer between collector and subcollector for the gettering of hydrogen that is implanted for the insulation of transistors.
DPSA (double polysilicon self-aligned) transistors are known e.g. from T. F. Meister et al., IEDM Technical Digest 1995, p. 739-741.
Their name stems from the fact that they use as p+-type base terminal and as n+-type emitter contact two p+-type polysilicon and n+-type polysilicon layers, respectively, which are deposited especially therefore. In this case, in the emitter window, the n+-type polysilicon emitter layer is insulated from the p+-type polysilicon layer of the base terminal in a self-aligned manner by means of a spacer. The DPSA transistor may contain both an implanted Si base and an epitaxially deposited SiGe base. The collector is usually connected via a buried layer that is buried in the substrate (also referred to as subcollector). On account of their lateral and vertical scalability and the low parasitic capacitance and resistance components the DPSA transistor structure is best suited to very high speed applications.
In order to make bipolar transistors suitable for ever higher frequencies, parasitic capacitances and resistances have to be reduced and charge carrier transit times have to be reduced. In the case of vertically constructed bipolar transistors, short transit times are achieved by means of very thin layers of emitter, base and collector.
A minimum diffusion of the dopants is crucial, because the layer thickness of the boron-doped base in the case of the npn transistor can only be chosen to be small enough that an outdiffusion of the boron into adjacent layers still does not entail any disadvantages. This is a central problem in the case of the npn bipolar transistor, because boron, besides the normal diffusion, additionally exhibits an accelerated diffusion induced by interstitial atoms.
Similar problems occur in the case of a phosphorus-doped collector, because phosphorus also has an accelerated diffusion through interstitial atoms which has the consequence that a doping profile present in the collector may be altered in an unintentional manner.
It is known that interstitial atoms arise e.g. as a result of oxidation and implantation. In particular, however, the deactivation of arsenic in highly doped layers (concentration of typically 1020 cm−3), such as e.g. in a buried subcollector layer in the bipolar transistor, also represents a source of interstitial atoms. In this respect, see P. M. Rouseau et al., Appl. Phys. Lett. 65 (5), 1995, “Electrical deactivation of arsenic as a source of point defects”. Since interstitial atoms are highly mobile in silicon, even more remote sources can induce an accelerated boron or phosphorus diffusion and thus make it impossible to miniaturize the transistor geometry further. Moreover, it is thus not practical to reduce the sheet resistance of the buried subcollector layer by means of a higher arsenic doping owing to the increase in disturbing interstitial atoms.
Reducing the concentration of the interstitial atoms is of crucial importance therefore for the further improvement of the high-frequency properties of bipolar transistors.
In principle, the concentration of the interstitial atoms can be reduced in two ways. Firstly, interstitial atoms can be prevented from arising through corresponding process control, and, secondly, interstitial atoms that have arisen can be annihilated again.
Interstitial atoms that have arisen are annihilated for example by using carbon-doped silicon layers which serve as a sink for interstitial atoms. This principle is successfully applied for example in the SiGe:C heterojunction bipolar transistor, as described e.g. in A. Gruhle et al., Appl. Phys. Lett. 75(5), 1999, “The reduction of base dopant outdiffusion in SiGe heterojunction bipolar transistors by carbon doping”.
Carbon doping of this type may usually be employed in this case in the base and/or in the emitter and/or in the collector as described in WO 98/26457 and US-2002/0,121,676 A1.
What is disadvantageous about the known procedure is that protection against accelerated diffusion is not completely possible, and, although it is possible to increase the effectiveness by means of higher carbon concentrations, this entails a degradation of the transistor characteristic curves and reduced charge carrier mobilities.
In particular, it is not possible to prevent interstitial atoms from arising from a buried subcollector region highly doped with arsenic because many indispensable process steps take place in the corresponding temperature range of between 500° C. and 900° C.
The only solution that has remained hitherto, therefore, has been merely to reduce the arsenic concentration in the buried subcollector region, which entails a disadvantage in that the sheet resistance of the subcollector region is increased and, accordingly, the high-frequency suitability of the transistor is impaired.